I have washed a quilt in the bathtub....so I have more space to let it spread out...tepid water, gentle soap, like the idea of baking soda....and just adgitate with your hands... I would wash an old top before trying to back and quilt...if seams come apart easier to repair and if it falls apart, you haven't invested all that time and effort.
It will also smell so much better during the quilting process.